This invention relates to a hard sintered alloy having the surface on which fine pores are formed or can be formed, and a process for preparing the same, more specifically to a hard sintered alloy having fine pores suitable for cutting tools such as insert tip, a drill and an end mill, plastic working tools such as a drawing mold, a die mold and a forging mold, shearing tools such as a punching tool and a slitter, and sliding materials such as mechanical seal and a bearing, and a process for preparing the same.
Hard sintered alloys such as a hard metal, a TiC/TiN-based cermet, a boride-based cermet, ferro-TiC and high-speed steel by powder metallugy, which are obtained by sintering hard powder such as WC, TiC, TiN, VC and MoB and metal powder such as Co, Ni and Fe according to powder metallugy, have excellent strength, toughness and wear resistance so that they have been widely used as various structural parts represented by cutting tools, wear parts and sliding materials. In cutting of an aluminum alloy, a Ti alloy and stainless steel which are easily welded, plastic working in which remarkable damage is caused by contact bonding of a material to be processed and a bearing with high precision which requires low rotary torque even at high surface pressure, a working solution or a lubricating oil have generally been used in order to ensure wear resistance, seizure resistance and lubricity. Even in these uses, there are required to effect high-speed processing, increase efficiency and elongate a life, but improvement of a working solution or a lubricating oil alone cannot cope with these demands.
Therefore, there have been proposed techniques of reducing friction and wear by dispersing pores in a sintered hard metal and impregnating the pores with lubricating oil or a solid lubricant, and representative examples thereof are described in Nishimura et al., "Powder and Powder Metallurgy", 36 (1989), 105 and Japanese Patent Publication No. 1383/1988.
Among the conventional techniques, in a hard metal for sliding of Nishimura et al. in which pores are dispersed, a spherical resin is added to starting powder and the resin is volatilized during sintering under heating to form dispersed pores. In this hard metal of Nishimura et al., pores are dispersed uniformly, but there are problems that the average diameter of the pores is large and remarkably fluctuated and the pores are formed from an inner portion to a surface portion of the hard metal so that strength and hardness are low and its application is limited. Further, the pores formed by volatilization of the resin during sintering under heating disappear as sintering proceeds so that there is also a problem in manufacture control that it is difficult to control the amount and average diameter of the pores.
On the other hand, Japanese Patent Publication No. 1383/1988 discloses an iron based sliding material in which surface portion voids at a depth of 1 mm from the sliding surface comprising 5 to 50% by weight of TiCN and the balance of an iron alloy is 7 to 20% by volume and an inner voids is made smaller than said ratio. When the iron based sliding material described in the above patent publication is used under conditions of using lubricating oil, the voids at the surface portion are impregnated with the oil to reduce friction and wear to a great extent. However, there are problems that it is extremely difficult to control the amount and size of the voids in press molding and sintering steps in powder metallugical techniques and it is also extremely difficult to make the voids remain only at a very surface portion so that strength and hardness are low and its application is limited.